It has previously been suggested to use a phase change material in a container within a thermally insulated case with the phase change material having heat exchange with an object within the container to maintain the object either warmer or colder than the ambient. Examples of containers using phase change material are disclosed in U.S. Pat. Nos. 3,255,607 to Bair et al and 4,019,340 to Conklin. Another example of a thermal energy storage device in which a material absorbs or releases heat to a given region as the material changes state is disclosed in U.S. Pat. No. 3,976,584 to Leifer.
The aforesaid Conklin patent has a phase change material supported within a cover of a thermally insulated case and exposed to objects within the case. The aforesaid Bair et al patent has a phase change material supported in a hollow space within a shelf dividing the container into two parts with objects above and below the shelf. The aforesaid Leifer patent contemplates using a thermal energy storage material in a thermally conductive sealed container and disposing the sealed container in heat transfer relation with the air or the region that is being thermally conditioned.
In each of the portable cases in the aforesaid Conklin and Bair et al patents, the heat transfer through the thermally insulated walls of the case can be substantially greater than the heat transfer between the phase change material and an object within the case to be protected if there is a significant temperature differential between the temperature of the phase change material and the ambient. Accordingly, the temperature of an object within the case can decrease substantially below the temperature of the phase change material in a relatively short period of time when the temperature of the ambient is substantially colder than the temperature of the phase change material or can increase above the temperature of the phase change material in a relatively short period of time when the temperature of the ambient is substantially higher than the temperature of the phase change material.
The reason for the heat transfer through the thermally insulated walls being greater than the heat transfer from the phase change material to an object to be protected is because heat transfer occurs faster where there is the greatest temperature differential. Thus, if the temperature differential existing between the ambient and the object to be protected is relatively large in comparison with the temperature differential between the phase change material and the object to be protected, there is a substantial heat transfer between the object to be protected and the ambient so that the temperature of the object to be protected approaches the ambient temperature rather than the temperature of the phase change material.
The aforesaid Leifer patent primarily discusses using a thermal energy storage material where there is either heating or cooling also being supplied at the same time. However, the aforesaid Leifer patent discusses that the thermal energy storage material could be disposed to insulate a storage or transportation compartment containing goods but does not state how it is disposed.